Mononuclear phagocytes can develop outside the marrow to gain markedly enhanced capacities for effecting a wide variety of functions. This development can be broadly termed activation. Over the past five years, the concept and study of macrophage activation have moved from descriptive phenomenology to detailed studies examining the regulatory and effector roles of specific molecules. It has become readily apparent that activation of mononuclear phagocytes is a complex, multistep event, which is induced by numerous signals and that activation, once induced, is further regulated by other signals, including secretory products from mononuclear phagocytes themselves. Furthermore, activation is accompanied by multiple changes in the mononuclear phagocytes, including alterations in the cell surface and in lytic functions. The time is thus ripe for a workshop where workers in the field can appraise and consolidate these advances and make effective plans to exploit these developments for future endeavors. At the same time, the study of the activation of mononuclear phagocytes is now somewhat hampered by problems of nonuniform semantics, techniques, and reagents. The technology to resolve these problems, however, is now at hand, if the appropriate workers address them effectively and develop useful plans for standardization and collaboration. A meeting on macrophage activation has not been held since 1975 and the need for such a meeting is widely and strongly recognized among workers in the field. We, therefore, propose to hold workshop on macrophage activation, which will: 1) facilitate the interchange of ideas and new data; 2) define the current stage of the field and areas for future endeavor; 3) resolve semantic and technical problems; 4) foster closer and more effective interaction between laboratories; and 5) promote standardization and exchange of techniques and reagents.